Two bomb squad detectives placed their ears against a leather suitcase.

Tick tock. Tick tock. Tick tock.

One of the detectives, Joe Lynch, bent down, removed a pocketknife and carefully cut a two-inch strip into the case. His longtime partner Freddy Socha knelt down beside him to peer inside, at what appeared to be dynamite.

Tick tock. Tick tock. Tick tock.

“This looks like the real goods,” Lynch said.

Suddenly, the two detectives were thrown into the air. The two were so mutilated by the explosion that they were no longer recognizable. And a city was suddenly terrified of terrorist attacks, gripped by a “see something, say something” vigilance.

It was New York’s first terrorist attack, thought, but never proved, to have been planned by Nazi sympathizers.

The date was July 4, 1940. The place was New York’s first World’s Fair, dubbed “The World of Tomorrow,” located in Flushing Meadows-Corona Park in Queens.

Yet few who pass by the remnants of this fair, and its 1964 cousin, know of the bombing. Certainly James Mauro, author of a new history of the fair, “Twilight at the World of Tomorrow,” hadn’t known about it until he stumbled upon a nondescript plaque buried outside the Queens Museum of Art.

On the plaque was the inscription: “This plaque is dedicated to the memory of detectives Joseph J. Lynch and Ferdinand A. Socha, Bomb and Forgery Squad, who were killed in the line of duty while examining a time bomb taken from the British Pavilion of the World’s Fair in Flushing Meadow Park at 4:45 p.m. on July 4, 1940.”

“There was a bomb at the World’s Fair? On the Fourth of July?” Mauro was dumbfounded. “I went home and immediately started looking into the fair and found it was a bigger and bigger story than I had ever imagined.”

THE ‘VALLEY OF ASHES’

Mauro would come to find a few more shocking details while rifling through the New York Public Library’s World’s Fair collection.

The idea for the World’s Fair started with an out of work engineer and his 12-year-old daughter. In 1934, Joseph Shadgen debated with his daughter Jacqueline about when the country was founded when it hit him: the US would be 150 years old in five years. What better way to celebrate than a World’s Fair he thought.

“Why don’t you do it daddy?” Jacqueline said, according to the book.

Shadgen hooked up with Edward Roosevelt, a cousin of Franklin and Eleanor, and pitched the idea to bigwig New York politicians.

But as always it was location, location, location. For the nation’s biggest World’s Fair — one which would put Chicago’s “A Century in Progress” to shame — they needed space.

Shadgen suggested the least likely of places, a smelly 1,200 acres of swamp and wasteland in Queens that was a stone’s throw from his Jackson Heights house. The “Corona Dumps,” as they were called, were made famous by F. Scott Fitzgerald’s “Great Gatsby.” The author named these hills “a valley of ashes — a fantastic farm where ashes grow like wheat into ridges and hills and grotesque gardens.” A trash heap isn’t necessarily the first place you’d think of when waxing poetically about the “future” and the “progress of nations.”

But the list of names who championed it reads like a history book roster of who’s who — Robert Moses, Mayor La Guardia, Albert Einstein (brought on as science advisor to the Fair) and even President Roosevelt saw potential in those ash heaps. Moses, who was already famous for developing Jones Beach, made it his life’s mission to create a park out of the festering Corona Dumps.

They needed money — and that’s where Grover Whalen, who was named the president of the World’s Fair Corporation, came in.

He was an outright dandy, complete with a white carnation, who brought a personal barber’s chair with him to every office he held, who seemed out of place in his undeserved role as police commissioner. But as the president, he seemed to fit right in.

After securing huge names like Guggenheim and Cartier as investors, he looked overseas to sign $30 million from 62 nations to be represented at the fair, the most ever, of which 22 agreed to build pavilions.

But to accomplish this — in the midst of war — took some sneakiness.

The first order of business was to get Russia involved.

Whalen wined and dined Constantine Oumansky, an advisor to the Soviets, and the Russian was so taken by the dandy that he called the Kremlin directly.

Over the phone, after only a half-hour of deliberating and without even arguing the price, Joseph Stalin himself agreed to erect a $4 million building, the Fair’s first large-scale foreign contract.

“Now every country would have to participate on a large scale. No one was going to allow the Russians to overshadow them at Flushing,” Whalen said, according to the book.

Next, he flew to Italy to see Il Duce.

Mussolini stood on a foot-high platform with his back to Whalen and broke the silence. “I understand you served as Police Commissioner of New York,” he said. “How did my people behave?”

“Some good, some bad,” Whalen responded.

“The bad ones — from Sicily?”

Mussolini first balked at participating in the Fair.

“What, Italy compete with Wall Street?” the dictator said. “What, for example, would it accomplish?”

“The American people would like to know what fascism is,” Whalen responded.

“You want to know what fascism is like? It is like your New Deal!”

Not easily sidetracked, Whalen tried flattery and won over the Tiny Terror, who agreed to pay for a $5 million pavilion.

THE WORLD OF TOMORROW

The price tag for the Fair? A whopping $160 million (nearly $2.3 billion adjusted for inflation) that seems almost preposterous considering that the United States was in the midst of the Great Depression.

Sure, it might have been in poor taste, but Whalen argued that it would bring in over a billion Depression-era dollars via tourism. Boy would he be wrong.

It would bring in some cutting-edge products, such as the first time the facsimile was introduced, and many zany ideas, like a chain-smoking robot.

Television made its first formal premiere in the United States during the Opening Ceremony on April 30, 1939. In a broadcast about the fair, FDR “declared it open to all mankind.” Nylon, 3-D movies and fluorescent lights were also among the more prominent inventions.

Air conditioning, which was rare at the time, was introduced to the masses at the Carrier Corporation’s “Eskimo Igloo of Tomorrow.” Inside the air-conditioned pavilion there were two thermometers: one reading the outside temperature, the other reading the inside one. By popularizing the idea of home air-conditioning, many cooling units were installed in private houses by the end of the war.

But the long list of ridiculous projects are far more entertaining: an outdoor section devoted to “Uninhibited Voodoo Dancers” a fancy name for “exotic” topless women, an “Obscene Octopus” a rubber sea animal that strips the bathing suits off female swimmers and “Pedro the Voder,” a synthetic human speech device developed by AT&T.

General Motors spent a massive $7 million ($100 million today) on a Futurama exhibit, featuring a ride through their vision of the world of 1960. The 18-minute ride simulated a flight over miles of not-so-outlandish American landscape, complete with a 14-lane superhighway (featuring lanes with speed limits as high as 100 mph), skyscrapers domed in glass and flying commuter aircrafts.

THE WAR INTRUDES

But among all these visions of progress — both silly and serious — the real world was looming. By Opening Day, Nazi Germany had occupied two countries, Austria and Czechoslovakia. And by the Fair’s end in 1940, Belgium, Denmark, France and the Netherlands would fall under Nazi control. The visions for the future were no longer so bright.

“The biggest irony of it is that men like Grover Whalen honestly believed that they were building this beautiful world of tomorrow as a showcase for what the world could be and as a coming together of nations,” Mauro said.

Mauro points to one fact that sums up the sad irony of the Fair’s vision versus reality: 20,000 tons of steel used to erect most of the Fair’s most iconic buildings were melted down to make guns, airplanes and battleships.

Furthermore, the Fair was considered by many to be a financial folly, falling $700 million short of its billion-dollar new business estimate.

But what about the bombing? In the 1930s, bombings were almost a way of life as warring unions terrorized movie theaters and public places with makeshift bombs that were most often terrifying nuisances, rather than lethal threats

However, during the World’s Fair, these bombings would become more significant in light of the war. Bomb threats were hardly ever taken seriously until the July 4th bomb went off, killing two police officers and wounding several others.

“That not only set off a bomb fever, but also a sense of Oh my God the war has come here,’ ” said author Mauro.

The US, still in the midst of fervent isolationism as the war waged overseas, had finally come to the realization, with this one bomb, that they were not safe. Shortly after the July 4 bombing, a Nazi flag was found in the same location as the ticking suitcase. The message was loud and clear.

A little over a year later, America would be hit with yet another attack — Pearl Harbor — thereby burying the memory of the two brave NYPD officers on a plaque often covered in brush and old leaves outside the old fairgrounds in Queens as the country prepared to enter the war.

To this day, there is still a reward, $26,000, to find the perpetrators of the forgotten crime.